Then you mixed it wrong. I've been using it for 20-years in both my longrifles and never had a problem. I did a LOT of reenactments from about 1999 through 2015 in both Vermont and Virginia, where I would typically fire 50-120 blank rounds per weekend. Not so much here in Washington state because there aren't very many opportunities.
Of course we use blank rounds at the reenactments. Blank rounds foul a barrel a LOT more than live rounds with ball and patch to wipe the barrel when you fire, so I always cleaned my rifle at the end of the day and I always used MAP mixed exactly as I posted. I was shown how to do that by a flintlock gun builder in our unit. I've never had any rust problems with either of my longrifles. Before I pickled the barrel on my first longrifle (Traditions Pennsylvania Longrifle) it had a blued finish and I never had any trouble with the bluing. There's really nothing caustic enough in that mixture to eat through a blued finish so I have no clue what your problem was. Sorry to hear about it.
Nonetheless, water and dish soap works or just plain water by itself, and if that's what you like to use, more power to you. Use what you like. The OP's question was on mixing, storing and using it, not what else to use. So I tried to answer his question.
